Phase Fluctuations Research Articles

Comparison of the Occurrence Morphology of Phase Scintillation of GPS and Beidou Signals at Zhongshan Station, Antarctica

AbstractThe characteristics of phase scintillation (represented by the phase scintillation indices, σφ) from GPS and Beidou are statistically analyzed using a ground‐based receiver at Zhongshan Station, Antarctica, from 2020 to 2022 for the first time. The phase scintillation of the GPS and Beidou signals present a similar pattern of occurrence. The statistical results on the occurrence morphology of phase scintillation show that the phase scintillation predominately occurs in the magnetic pre‐noon and pre‐midnight sectors. Moreover, phase scintillation performs a dependence on solar and geomagnetic activities. Furthermore, the phase scintillation also gives a seasonal variation with the maximum occurrence happened at the autumn and the minimum occurrence during the summer. Consequently, these results improve understanding of the morphological characteristics of the phase fluctuations in the less studied Antarctic region. The study also demonstrates the use of the combined data set to improve the coverage in the Antarctic region.

Overcoming laser phase noise for low-cost coherent optical communication

Artificial-intelligence-generated content has driven explosive data traffic growth in data-center interconnects. Traditional direct detection solutions struggle with limited spectral efficiency and distance, prompting the shift to coherent optics for cost-sensitive short-reach links. One specific challenge is integrating low-cost lasers while overcoming severe phase noise on high-order modulation formats. Here, we propose a residual carrier modulation scheme for precise and efficient carrier frequency and phase recovery. The residual optical carrier can continuously track phase fluctuations without redundancy compared with discrete time-domain pilots, and address the digital-to-analog convertor resolution reduction issue of frequency-domain digital pilots. In proof-of-concept experiments, we transmit a net 1-Tb/s probabilistic-shaped 256-ary quadrature amplitude modulated (PS-256-QAM) signal using a 3 MHz distributed feedback (DFB) laser. Our scheme improves bitrate by 41% compared to conventional time-domain pilots, achieving a record laser linewidth sum and symbol duration product of 6.89 × 10−5. This approach supports MHz linewidth DFB lasers in low-cost coherent optical communications.

Bayesian approach to coherent combination of single photon beams

We theoretically investigate the performance of a coherent beam combination of two light beams under relative phase fluctuations in the photon starved regime. We apply a first-principles approach using the optimal Bayesian phase correction protocol. We analyze the efficiency of beam combination as a function of the phase fluctuations strength.

Anomalous Superfluid Density in a Disordered Charge-Density-Wave Material: Pd-Intercalated ErTe_{3}.

We image local superfluid density in single crystals of Pd-intercalated ErTe_{3} below the superconducting critical temperature T_{c}, well below the onset temperature T_{CDW} of (disordered) charge-density-wave order. We find no detectable inhomogeneities on micron scales. We observe a rapid increase of the superfluid density below T_{c}, deviating from the behavior expected in a conventional Bardeen-Cooper-Schrieffer superconductor, and show that the temperature dependence is qualitatively consistent with a combination of quantum and thermal phase fluctuations.

Using the phase characteristic for source depth discrimination with a horizontal line array for a broadband source

A solution to the problem of source depth discrimination in a shallow water waveguide is proposed for the case of a horizontal line array and low-frequency broadband sources. The method divides the horizontal line array into two subarrays with the same number of elements and then calculates the cross-power spectrum of beamforming pressure outputs of the subarrays. Depth-dependent features of phase fluctuation in the cross-power spectrum are analyzed using longitudinal correlation, and depth-separable decision metrics are defined. These decision metrics are poorly impacted by the energy distribution of the signal spectrum. Depth discrimination relies on the phase variation of the cross-power spectrum, and thus does not require mode space information or geoacoustic parameters as the prior knowledge, but sufficient effective horizontal apertures. Due to the application of beamforming, array gain can be obtained to improve the input signal-to-noise ratio of the phase feature-based discriminators. Simulations and Monte Carlo methods are applied to performance predictions and method comparison. The impact on the performance of signal-to-noise ratio, horizontal aperture, and array overlap is numerically studied. Finally, the method is experimentally demonstrated with data from a bottom-mounted horizontal line array deployed in the South China Sea, and the applicability of array overlap is illustrated.

Quantum fluctuation in the segregation phase of two-component weakly interacting Bose–Einstein condensates

Our investigation focuses on the condensate density, excited and condensed fractions of bosons in a mixture of two-component segregated weakly interacting Bose–Einstein condensates (BECs) at zero temperature based on the Gross–Pitaevskii (GP) and Cornwall-Jackiw-Tomboulis (CJT) theories. We find that the repulsive interaction among interspecies of bosons is the origin of the local behavior and the decline of quantum fluctuation. These features are disparate to the single BEC or miscible BECs. The numerical evaluation of the condensed fraction is provided to support the experimental observation.

Modified compressible turbulence phase distortion formula and compressible turbulence spectrum simulation

In order to better study the imaging characteristics of high-speed moving targets in atmospheric turbulence, it is necessary to solve the optical distortion caused by aerodynamic optical effects, and the optical distortion needs to be described by compressible turbulence spectrum. In order to better understand the spectral characteristics of aero-optical distortion, according to the Maxwell equation, the Boltzmann factor equation is introduced based on the existing compressible turbulence spectrum theory, and the analytical expression of the modified compressible turbulence spectrum is derived and verified. Then, based on the freezing theory, the compressible turbulence spectrum is simulated by the phase screen method of power spectrum inversion. The influence of the distance from the moving target wall and turbulence intensity on the compressible turbulence phase screen is simulated and analyzed. Compared with atmospheric turbulence, it is found that the intensity of turbulence is proportional to the compressible turbulence, but the compressible turbulence is also inversely proportional to the distance of the moving target wall. As the distance from the moving target wall decreases, the phase fluctuation of the compressible turbulence is larger, which also indicates that the aero-optical effect mainly occurs in the boundary layer annex of the moving target, and the nearer the target, the aero-optical effect becomes more serious.

Scintillation Level of Scattered Radio Waves in the Equatorial Ionosphere

Scintillation effects of propagating and scattered radio waves (RWs) are considered analytically and numerically in the equatorial terrestrial ionosphere applying the statistical characteristics. Analytical calculations of the second-order statistical moments of the phase fluctuations of the ordinary and extraordinary RWs in the conductive collision ionospheric magnetoplasma (COCOIMA) are carried out for the first time using the Wentzel-Kramers-Brilluen method and the modified smooth perturbation method in consideration of the diffraction effects. Polarization coefficients of these waves in the equatorial ionosphere have been derived for the first time. The scintillation level includes the variance and the correlation functions of scattered RWs. Statistical moments contain complex refractive index, anisotropy coefficient characterizing irregular plasmonic structures, tilt angle of prolate irregularities concerning to the external magnetic field, permittivity components of the equatorial ionosphere, Hall's, Pedersen and longitudinal conductivities. A new feature of the "Fountain Effect" has been discovered in the equatorial ionosphere at weak and moderate scintillations caused due to anisotropy parameters of electron density fluctuations. Anisotropy parameters of electron density inhomogeneities influence the scintillation index of both the ordinary and extraordinary radio waves shifting maximums of the curves in the opposite directions at weak and moderate scintillations. Numerical calculations are carried out using the hybrid anisotropic correlation function of electron density fluctuations containing both exponential and power-law spectral functions having arbitrary spectral index, applying the experimental data.

Far‐Field Groundwater Response to the Lamb Waves From the 2022 Hunga‐Tonga Volcano Eruption

AbstractOn 15 January 2022, the largest eruption of the Hunga‐Tonga volcano in recorded history produced a plume registered by multi‐parametric instruments around the world. However, the far‐field hydrogeological responses to Lamb waves from this eruption remain underexplored. We studied the responses of groundwater to the volcanic eruption in the far‐field over 8,700 km, including 274 wells. Results show that the Lamb waves with a speed of 316 m/s affects the groundwater system, leading to similar fluctuations in well water level (WL) and opposite phase fluctuation in borehole strain. Different wells exhibit diverse responses in WL amplitudes, possibly for heterogeneities in local aquifer systems. Gain values of 5 wells that simultaneously measure atmospheric pressure, borehole air pressure, borehole strain and WL are consistent with results obtained through cross‐power spectrum estimation. This work demonstrates a novel response in far‐field groundwater systems induced by Lamb waves and expects application for aquifer parameter estimation.

The nematic susceptibility of the ferroquadrupolar metal TmAg2 measured via the elastocaloric effect

Elastocaloric measurements of the ferroquadrupolar/nematic rare-earth intermetallic TmAg2 are presented. TmAg2 undergoes a cooperative Jahn-Teller-like ferroquadrupolar phase transition at 5K, in which the Tm3+ ion’s local 4f electronic ground state doublet spontaneously splits and develops an electric quadrupole moment which breaks the rotational symmetry of the tetragonal lattice. The elastocaloric effect, which is the temperature change in the sample induced by adiabatic strains the sample experiences, is sensitive to quadrupolar fluctuations in the paranematic phase which couple to the induced strain. We show that elastocaloric measurements of this material reveal a Curie-Weiss like nematic susceptibility with a Weiss temperature of T* ≈ 2.7K, in agreement with previous elastic constant measurements. Furthermore, we establish that a magnetic field along the c-axis acts as an effective transverse field for the quadrupole moments.

Adiabatic weak coherent MHz linewidth O-band single-photon carrier for low erroneous phase decoding

For weak coherent single-photon secure data communication among short-reach metropolitan intra-/inter-city networks at the O-band (1250-1350 nm), the commercially available semiconductor laser sources are emerging but still suffering from high single-mode-fiber (SMF) loss, broad linewidth, and unstable wavelength. To overcome such disadvantages for enabling the efficient phase-coding link with sufficient secure key rate, a specifically designed adiabatic package with active temperature-/current-feedback control is proposed for the paired O-band MHz-linewidth master-to-slave injection-locked DFBLDs and a polarization-maintaining 1-bit-delay interferometer is stabilized with using a passively adiabatic cell to achieve accurate differential phase decoding. Even though, the phonon-induced phase fluctuation still occurs at rising and falling edges of the decoded long-pattern secure data bits delivered from the slave DFBLD, which is mainly attributed to the intra-cavity heating under excessive free-carrier generation via the master DFBLD injection. To stabilize the differential-phase-shift (DPS) keying protocol, the phase-code distortion caused by over-injection-induced Auger heating is effectively suppressed by reducing the overly biased injection with precise master-injection-level control. The rising-/falling-edge damping distortion of the phase-shift-encoded secure bit-stream envelope is suppressed by appropriately decreasing the DC bias current and adjusting the AC encoding amplitude of the master DFBLD. Such operation reduces the incorrect π phase shift in the injection-locked slave DFBLD biased at optimized below-threshold DC offset, thus allowing single-photon DPS-keying data transmission over 15-km SMF with slightly increasing the single-photon bit-error ratio from <3% (0-km) to 6.2% (15-km).

Phase noise suppression of optic flexural disk accelerometer by studying the thermal stability of optical fiber ring

As the core sensing elements of ultra-long fiber interferometer, the distributed thermal strain difference of the fiber rings can cause extra noise of the flexural disk, resulting in a penalty of the deterioration accuracy. In this paper, the thermal strain distribution characteristics of the fiber ring are firstly analyzed by the finite element method (FEM), and the distribution result is consistent with that demonstrated by the Rayleigh optical frequency-domain reflectometry (R-OFDR) strain measurement. The interferometer phase noise caused by the distributed strain difference is further studied by constructing a fully symmetric polarization-maintaining fiber-ring Mach-Zehnder interferometer (MZI) with an arm length of over 100 meters. The results show that the distributed thermal strain difference of two fiber rings will cause additional phase fluctuation, which leads to higher low-frequency noise. Therefore, a dual-fiber-ring MZI with matched distributed thermal strains is proposed to suppress the phase noise caused by the thermal strain, and the best suppression is as high as 45.6 dB. This is very important for the research and design of low noise fiber seismometer.

Features of Adaptive Phase Correction of Optical Wave Distortions under Conditions of Intensity Fluctuations

An analysis of the features of measurements and correction of phase distortions in optical waves propagating in the atmosphere at various levels of turbulence was performed. It is shown that with increasing intensity fluctuations, the limiting capabilities of phase correction decrease, and the phase of an optical wave that has passed through a turbulence layer consists of two components: potential and vortex. It was found that even in the region of weak fluctuations there is an overlap of spectral filtering functions for intensity and phase fluctuations. Areas of turbulence inhomogeneities have been identified that will have mutual influence and negatively affect the operation of the phase meter. It is noted that correlation functions, both phase and intensity, are less susceptible to this compared to structural functions. The results of experimental studies on the reconstruction of the wavefront of laser radiation distorted by atmospheric turbulence using a Shack–Hartmann wavefront sensor during vignetting and central screening of the entrance pupil in the optical system are presented. Studies have been carried out on the propagation of laser radiation along a horizontal atmospheric path for various levels of turbulence. The results are analyzed in terms of Zernike polynomials.

Effect of multi wall carbon nanotubes during semi-solid to solid phase transformation in eutectic Al-Si alloy

The study explores numerically the combined effect of the base metal pouring temperature and MWCNT (multi wall carbon nanotubes) during semi-solid to solid solidification of Al-Si-MWCNT composite material. The semi-solid to solid solidification actually differs significantly both qualitatively and quantitatively from the liquid to solid solidification. During solidification of Al-Si-MWCNT composite material, the melt temperature and the cooling rate play crucial roles in the phase transformations. During liquid to solid solidification, the MWCNT significantly affects the cooling rate, leading to multiple recalescence phenomena, and influences the formation and evolution of the columnar and globular grains. Under the same condition, the MWCNT also impacts the silicon segregation, causing fluctuations in the silicon phases and its redistribution. However, during the semi-solid to solid solidification, it is observed that by decreasing the melt temperature into a semi-solid state, the segregation rate of silicon decreases which enhances more homogeneous CNT distribution. It also increases the globular volume fraction and dominates the columnar growth. Overall, the future trend of semi-solid to solid solidification of Al-Si-MWCNT composite materials lies in optimizing their properties for specific applications, expanding their use across diverse industries, and promoting sustainable manufacturing practices.

Single-shot diffusion trace spectroscopic imaging using radial echo planar trajectories.

Demonstrate the feasibility and evaluate the performance of single-shot diffusion trace-weighted radial echo planar spectroscopic imaging (Trace DW-REPSI) for quantifying the trace ADC in phantom and in vivo using a 3T clinical scanner. Trace DW-REPSI datasets were acquired in 10 phantom and 10 healthy volunteers, with a maximum b-value of 1601 s/mm2 and diffusion time of 10.75 ms. The self-navigation properties of radial acquisitions were used for corrections of shot-to-shot phase and frequency shift fluctuations of the raw data. In vivo trace ADCs of total NAA (tNAA), total creatine (tCr), and total choline (tCho) extrapolated to pure gray and white matter fractions were compared, as well as trace ADCs estimated in voxels within white or gray matter-dominant regions. Trace ADCs in phantom show excellent agreement with reported values, and in vivo ADCs agree well with the expected differences between gray and white matter. For tNAA, tCr, and tCho, the trace ADCs extrapolated to pure gray and white matter ranged from 0.18-0.27 and 0.26-0.38 μm2/ms, respectively. In sets of gray and white matter-dominant voxels, the values ranged from 0.21 to 0.27 and 0.24 to 0.31 μm2/ms, respectively. The overestimated trace ADCs from this sequence can be attributed to the short diffusion time. This study presents the first demonstration of the single-shot diffusion trace-weighted spectroscopic imaging sequence using radial echo planar trajectories. The Trace DW-REPSI sequence could provide an estimate of the trace ADC in a much shorter scan time compared to conventional approaches that require three separate measurements.

Low-complexity continuous-variable quantum key distribution with true local oscillator using pilot-assisted frequency locking

In the domain of continuous variable quantum key distribution (CV-QKD), a significant challenge arises in achieving precise frequency synchronization, an issue commonly termed as frequency locking. This involves matching the optical frequencies of both the quantum signal laser and the local oscillator laser for accurate symbol demodulation during the exchange of quantum keys. As such, implementations today still grapple with maintaining precise synchronization between sender and receiver frequencies, occasionally hindering the efficiency and reliability of the information exchange. Addressing this challenge, we present and empirically validate a novel approach to CV-QKD by incorporating a pilot tone-assisted frequency locking algorithm to enhance stability when using a locally generated local oscillator (LLO) at the receiver. The proposed design leverages software-based optimization techniques, thereby eliminating the need for high-speed electronic stabilization devices and achieving efficient performance at typical repetition rates. Specifically, the introduction of the pilot tone algorithm allows us to effectively mitigate phase fluctuations and preserve the integrity of the quantum signals during transmission without resorting to time-multiplexed reference pulses or fast-locking electronics in the lasers. Our results suggest the potential for achieving secure key rates of up to 1 Mb/s over a 50 km single-mode fiber when using these techniques, offering promising insights into the feasibility of high-rate, low-complexity CV-QKD implementations under realistic conditions.

Spatial-to-spectral phase coupling mechanisms in bulk continuum generation

We study the coherence properties of continuum generation in YAG crystals seeded by 180 fs pulses at 1035 nm when the driving beam exhibits small fluctuations of the spatial phase. The relative stability of the continuum spectral phase is first assessed as a function of the driver wavefront aberrations. Furthermore, we evidence and quantify a coupling mechanism between these fluctuations and the spectral phase of the continuum. The coupling coefficients increase with the spectral broadening and are also unexpectedly large (up to tens of rad/rad at ≃ 750 nm). Experimental evidence supports that longitudinal shifts of the position of the filament within the crystal are responsible for such strong effects.

Power Spectral Characteristics of In‐Situ Irregularities and Topside GPS Signal Intensity at Low Latitudes Using High‐Sample‐Rate Swarm Echo (e‐POP) Measurements

AbstractIonospheric density structures at low latitudes range in size from thousands of kilometers down to a few meters. Radio frequency (RF) signals, such as those from global navigation satellite systems, that propagate through irregularities suffer from rapid fluctuations in phase and intensity, known as scintillations. In this study, we use the high‐sample‐rate measurements of the Swarm Echo (CASSIOPE/e‐POP) satellite's GPS Occultation (GAP‐O) receiver taken after its antenna was re‐oriented to vertical‐pointing, simultaneously with e‐POP Ion Mass Spectrometer surface current observations as a proxy for plasma density, to obtain the spectral characteristics of GPS signal intensity and in‐situ irregularities at altitudes from 350 to 1,280 km. We show that the power spectra of both measurements can generally be characterized by a power law. In the case of density irregularities, the spectral index with the highest occurrence rate is around 1.7, which is consistent with previous studies. Also, all the power spectra of GPS signal intensity in this study show a single spectral index near 2. Moreover, roll‐off frequencies estimated in this work range from 0.4 to 2.5 Hz, which is significantly higher than Fresnel frequencies calculated from ground GPS receivers at low latitudes (between 0.2 and 0.45 Hz). Part of this increase is due to the 8 km/s orbital velocity of Swarm Echo near perigee. Another key difference is that variations in the GPS signals in this study are dominated by the topside ionosphere, whereas GPS signals received from ground are affected mostly by the relatively dense F‐region plasma in the 250–350 km altitudinal range.

High-precision frequency-controlled optical phase shifter with acousto-optic devices.

A fundamental parameter to determine how electromagnetic waves interfere is their relative phase, and achieving a fine control over it enables a wide range of interferometric applications. Existing phase control methods rely on modifying the optical path length either by changing the path followed by the light or by altering the thickness or index of refraction of an optical element in the setup. In this Letter, we present a novel, to the best of our knowledge, method, based on acousto-optic modulators (AOMs), which allows adjusting the phase by shifting the frequency of the light in a segment of its path. Since the amount of phase shift depends on the length of the segment, an optical fiber is used to realize a 2π shift. Two experimental implementations are described which deal with different sources of phase fluctuations. The first addresses fluctuations resulting from the optical fiber, while the second tackles unwanted variations originating from the AOMs.

Scintillating and decorrelating signals for different propagation paths in a random medium

Randomness in the medium of propagation affects the signal received at a distance from a source resulting in amplitude and phase fluctuations as well as reduced coherence between different paths of propagation. A method is described and validated for incorporating such fluctuations and decorrelations in sound signals in a single approach. Here, simplified conditions are used of open space, with or without a single reflecting surface, assuming fluctuations as for plane waves in homogeneous and isotropic temperature turbulence. Validating results are shown for Gaussian and von Karman turbulence models and exemplifying sound clips are provided as supplementary material.